The importance of the symptom of itch in scalp dermatitis has been evaluated in the past. The most frequently described condition associated with unhealthy scalp is itch. This is also the most bothersome symptom, having the largest negative influence on sufferers' quality of life.
With regard to the general physiology of itch, the perception of itch represents the end of a complex physiological pathway that is initiated at the skin surface. A number of stimuli can start the cascade of events that eventually lead to the perception of itch. The stimulus binds to specialized receptor cells in the dermis triggering the release of histamine. Histamine then binds to the nerve endings, starting an electrical pathway along the nerve fiber that is transferred along the spinal column until it is received in the brain and interpreted as itch.
In general, while there is a wide range of potential mediators of itch, histamine is considered the prototypical physiological chemical mediator of itch. Histamine is a small molecule amine that was shown in 1927 to be a natural constituent of skin tissue (hence the name from the Greek word for tissue, histos). Key support for the direct role of histamine in itch is that antihistamines can be very effective in relieving itch in some skin conditions. Another argument in favor of the role of histamine is that it can be used to experimentally induce itch.
For specific cases, the strength of the association between histamine and itch has been studied for a number of specific skin conditions. There is a very strong association between histamine levels and itch severity for conditions such as urticaria and insect bites. The association with atopic dermatitis is less well-defined. Higher histamine levels have been found in the skin of sufferers of eczema and psoriasis. Even dry skin has been found to result in higher histamine levels.
While the histamine levels in dandruff and seborrheic dermatitis have not been previously investigated, it is likely there is a strong relationship between the itch severity and histamine levels. First, psoriasis and dandruff share many common etiological features, making it likely that the established histamine-itch relationship in psoriasis will apply to dandruff as well. It has also been demonstrated [Harding, C., A. Moore, J. Rogers, H. Meldrum, et al., Dandruff: a condition characterized by decreased levels of intercellular lipids in scalp stratum corneum and impaired barrier function. Arch. Dermatol. Res., 2002. 294: p. 221-230.] that dandruff sufferers have an increased incidence of itch in response to topically applied histamine, demonstrating the physiological relevance of histamine in scalp itch associated with dandruff.
Alternative biomarkers which may serve as mediators for itch are Substance P, and cannabinoids. Other possible mediators are provided in Neurophysiology of Pruritus; Cutaneous Elicitation of Itch; Arch Dermatol./Vol. 139, November 2003 and incorporated by reference herein.